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Your question is two-fold, I'm answering one part here: "my laptop is very slow". The sudden restarts should be asked in a different question.

Your hardware is relatively old and low-specced for this recent Ubuntu release. More specifically, you're probably hitting the slowness of Unity 3D. Since the 12.10 release the Unity 2D, meant for these devices, was removed. More on that topic is in this OMG!Ubuntu! article.

From here I believe you have two options:

Try one of the official derivatives of Ubuntu with a different desktop environment: Lubuntu (LXDE powered), Kubuntu (KDE powered), Xubuntu (XFCE powered). These are not using Unity and your desktop will look completely different.

There's no need to reinstall the current Ubuntu installation! Just install the lubuntu-desktop / kubuntu-desktop / xubuntu-desktop metapackage (for Lubuntu / Kubuntu / Xubuntu respectively - pick one) and you'll be able to choose the desktop environment to run at the login screen.

While KDE isn't as lightweight as XFCE and LXDE, I think it runs pretty well on older hardware with 2GB memory by installing the kubuntu-low-fat-settings package.

Install Ubuntu 12.04 with proper Unity 2D support. It's an LTS release (5 years support), so I'd recommend this if you only want to put effort in getting it to run once.